Aluminum is produced conventionally by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures between about 900 and 1000° C.; the process is known as the Hall-Heroult process. A Hall-Heroult reduction cell typically comprises a steel shell having an insulating lining of refractory material, which in turn has a lining of carbon that contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate that forms the cell bottom floor. The carbon lining and cathode substrate have a useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminum as well as intercalation of sodium, which causes swelling and deformation of the cathode carbon blocks and ramming mix. In addition, the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides. Anodes are at least partially submerged in the bath.
In operation, the conventional cell contains an electrolytic, molten cryolite-based bath in which alumina is dissolved. A molten aluminum pool acts as the cathode. A crust of frozen electrolyte and alumina forms on top of the bath and around the anode blocks. As electric current passes through the bath between the anode and cathode surfaces, alumina is reduced to aluminum, which is deposited in the pad of molten metal.
Electrolytic reduction cells must be heated from room temperature to approximately the desired operating temperature before the production of metal can be initiated. Heating should be done gradually and evenly to avoid thermal shock, which can in turn cause breakage or spalling. The heating operation minimizes thermal shock to the lining, the electrodes and the support structure assemblies upon introduction of the electrolyte and molten metal to the cell. Once at operating temperatures carbon anodes wear out and are replaced. Carbon anodes can be placed in to the electrolyte cold and heated by the energy of the cell to operating temperatures, at which time the nominal current of the anode will be attained. Ceramic anodes that have much longer lives are prone to thermal shock and therefore need to be preheated in a furnace outside of the electrolytic cell prior to insertion into the hot electrolyte. During transfer, the cooling or heating of the anodes must also be minimized to avoid thermal shock. The thermal shock/cracking can occur both during movement of the anodes into position and during their placement into the molten salt. Thermal shock relates to the thermal gradient (positive or negative) through the anode that occurs during the movement from the preheat furnace to the cell, and also upon insertion of the anodes into the molten salt. A thermal gradient as low as 50° C. can cause cracking. An attempt to protect electrodes in an electrolysis cell from thermal shock during start-up, U.S. Pat. No. 4,265,717, taught protection of hollow cylindrical TiB2 cathodes by inserting aluminum alloy plugs into the cathode cavity and further protecting the cathode with a heat dispersing metal jacket having an inside heat insulating layer contacting the TiB2, made of expanded, fibrous kaolin-china clay (Al2O3·2SiO2·2H2O), which would subsequently dissolve in the molten electrolyte.
Aluminum electrolysis cells have historically employed carbon anodes on a commercial scale. The energy and cost efficiency of aluminum smelting can be significantly reduced with the use of inert, non-consumable, and dimensionally stable anodes. Use of inert anodes rather than traditional carbon anodes allows a highly productive cell design to be utilized, thereby reducing capital costs. Significant environmental benefits are also realized because inert anodes produce essentially no CO2 or CF4 emissions. Some examples of inert anode compositions are provided in U.S. Pat. Nos. 4,374,050; 4,374,761; 4,399,088; 4,455,211; 4,582,585; 4,584,172; 4,620,905; 5,279,715; 5,794,112; 5,865,980; and 6,126,799 assigned to Alcoa Inc. These patents are incorporated herein by reference. Ceramic anodes, unlike their carbon predecessors, can undergo thermal shock and cracking if heated or cooled too quickly. Methods of protecting inert anodes from thermal shock are therefore desired.